A method of improving the photosensitivity of metal oxide semiconductors

ABSTRACT

This disclosure relates to a process of improving the photoconductivity of metal-containing semiconductors by heating the semiconductor at elevated temperatures and rapidly quenching the heated semiconductor. Particularly, the semiconductor is heated in an atmosphere which reversibly alters the stoichiometry of the semiconductor and then rapidly quenched in the atmosphere. The major improvement in the semiconductor lies in a substantial increase in the photographic speed of media comprising the semiconductor when exposed to activating radiation.

United States Patent Martin L. Hnrvlll Lexington, Mass. 706,186

Feb. 16, 1968 Dec. 28, 1971 ltek Corporation Lexington, Mass.

lnventor Appl. No. Filed Patented Assignee METHOD or IMPROVING THE 'PHOTOSENSITIVITY OF METAL OXIDE SEMICONDUCTORS 23 Claims, 3 Drawing Figs.

v.s.c1 96/88, 96/1 .5, 9611.8, 96/27, 252/501, 106/296, 106/300, 23/202 161. c1 G03g 5/02, 0056 1/72 FieldotSearch 252/501; 96/15, 1.8, 27, 88; 23/202; 106/296, 300

[56] References Cited UNITED STATES PATENTS 2,408,475 10/1946 Nickle 96/ 1.8 X 3,170,886 2/1965 Morrison et al. 252/501 3,409,429 11/1968 Ekman et a1 96/27 3,453,141 7/1969 Anolick et al. 252/501 X Primary Examiner-George F. Lesmes Assistant Examiner-R. E. Martin Attarneys- Homer 0. Blair, Robert L. Nathans and W. Gary Goodson semiconductor lies in a substantial increase in the photographic speed of media comprising the semiconductor when exposed to activating radiation.

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ATTORNEY.

METHOD OF IROYING Tl m PHOTQSENSITIVITY 01F METAL OXIDE SEMICONDUCTORS BACKGROUND OF THE INVENTION 1. Field of Invention This invention relates to processes for improving the photoconductivity of semiconductors and the products obtained thereby.

2. Description of Prior Art Data or image-storage media comprising radiation-sensitive materials such as titanium dioxide are described in detail in US. Pat. Nos. 3,152,903; 3,052,541 French Pat. Nos. 345,206 and 1,245,215 and in commonly owned copending US application Ser. No. 199,211, filed May 14, 1962 in the name of Elliot Berman et al. now abandoned. In the aforementioned u.S. patent application, a radiation-sensitive material, such as titanium dioxide, functions as a photosensitive component of the media and exposure of said media to activating means such as radiant energy, electron beams or the like results in the storage of a reversible latent image pattern therein. The reversible latent image pattern exists for a finite time during which said pattern can be converted to an irreversible form and read out visually by contacting said pattern with a suitable image-forming material, such as a chemical redox system. In the aforesaid U.S. and French patents, the radiation-sensitive material is combined with at least one component of an imageforming material prior to exposure to activating means. For example, US. Pat. No. 3,152,904 describes photosensitive copy media comprising a photosensitive material such as titanium dioxide in combination with a reducible metal ion such as silver nitrate. This copy media is exposed to activating means and then contacted with a reducing agent to produce a visible image. On the other hand, U.S. Pat. No. 3,152,903 discloses a system wherein the photosensitive material is used in combination with both oxidizing agent such as silver nitrate and a reducing agent such as hydroquinone. Upon exposure to suitable activating means, a visible image is formed. One of the limitations of the above-mentioned data or image storage systems is that they lack the photographic speed of systems such as silver halide. Therefore, in order to expand the possible uses of these photographic systems described in the abovementioned patents and application, it is highly desirable to increase the photographic speed of these systems. Much research effort has been spent in trying to find ways of increasing the speed of these systems. However, up to the time of the present invention, these efforts have met with little or no success.

SUMMARY OF THE INVENTION It has now been unexpectedly found that photographic response of metal-containing semiconductors may be substantially improved by heating the semiconductor, i.e. photoconductor, at elevated temperature and rapidly quenching the heated photoconductor. In a preferred form, the photoconductor is heated in an atmosphere which reversibly alters the stoichiometry of the photoconductor and then the heated photoconductor is rapidly quenched in the same atmosphere. The atmosphere is preferably a reducing atmosphere which contains oxygen and, during the heating step the photoconductor is reduced, i.e. the stoichiometry of the photoconductor is altered. After rapid quenching, the surface of the photoconductor chemisorbs oxygen on exposure to the atmosphere while the mass of the photoconductor remains substantially unchanged from the heat-induced nonstoichiometry. The chemisorption of the atmospheric oxygen is quite rapid at or near room temperature and equilibrium is rapidly attained, i.e. no further chemisorption of oxygen by the surface occurs and the surface of the photoconductor is equilibrated with oxygen.

For the aforementioned reducing atmosphere, oxygen is provided in the form of a mixture with other gases, such as carbon dioxide, or formed in situ as in mixtures of carbon monoxide and carbon dioxide, preferably at a fugacity of oxygen of less than one. The semiconductors, after treatment, show several fold increases in photographic speed. For example, samples of titanium dioxide showed increases in photographic speed of from two to four times an untreated sample.

DESCRIPTION OF PREFERRED EMBODIMENTS The photoconductors, or photocatalysts, preferred in this invention are metal-containing photoconductors. A preferred group of such photosensitive materials are the inorganic materials such as compounds of a metal and nonmetallic element of group VIA of the Periodic Table* (*Periodic Table from Langes HANDBOOK OF CHEMISTRY, 9th Edition pp. 56-57, 1966) such as oxides, e.g. zinc oxide, titanium dioxide, zirconium dioxide, germanium dioxide, indium trioxide; metal sulfides, e.g. cadmium sulfide (CdS), zinc sulfide (ZnS) and tin disulfrde (SnS,); and metal selenides such as cadmium selenide (CdSe). Metal oxides are especially preferred photoconductors of this group. Titanium dioxide is a preferred metal oxide because of its unexpectedly good properties. Titanium dioxide having an average particle size of about 250 millimicrons or less and especially, that titanium dioxide produced by high-temperature pyrolysis of titanium halide.

While the exact mechanism by which the photoconductors work is not known, it is believed that exposure of photoconductors or photocatalysts to activating means causes an electron or electrons to be transferred from the valence band of the photoconductor or photocatalyst to the conductive band of the same or at least to some similar excited state whereby the electron is loosely held, thereby changing the photoconductor from an inactive form to an active form. If the active form of the photoconductor or photocatalyst is in the presence of an electron-accepting compound a transfer of electrons will take place between the photographic and the electron-accepting compound. Therefore a simple test which may be used to determine whether or not materials have a photoconductor or photocatalytic effect is to'mix the material in question with an aqueous solution of silver nitrate. Little, if any, reaction should take place in the absence of light. The mixture is then subjected to light, at the same time that a control sample of an aqueous solution of silver nitrate alone is subjected to light, such as ultraviolet light. If the mixture darkens faster than the silver nitrate alone, the material is a photoconductor or photocatalyst.

If for example titanium dioxide is made nonstoichiometric by some means, it cannot readily become stoichiometric in its crystallite bulk at room temperature. For example, oxygen diffusion through the crystal lattice of titanium dioxide will not occur to a degree below the Tammann temperature which is about half the melting temperature in degrees Kelvin (780 C. for titanium dioxide). At room temperature in air, however, the overall equilibrium composition should be very nearly TiO m so that there is a tendency to become stoichiometric even though the equilibrium composition cannot be established throughout the bulk of the crystal. Therefore, the surface of the photoconductor can be returned to or near any desired, stoichiometry while the mass of the photoconductor remains substantially unchanged, i.e. the nonstoichiometry of the mass of the photoconductor created by heating is frozenin.

This nonstoichiometry yields electrons in or very near the conduction band so that they are relatively mobile, especially compared to oxygen ions. Therefore, oxygen from the atmosphere could possibly remove electrons from the conduction band and become chemisorbed to the surface yielding an overall composition which is nearly the equilibrium composition even through the oxygen could still not reach the interior of the crystal. This process would build up a potential difference between the interior and the exterior of each particle, and the potential difference would increase as nonstoichiometry of the bulk increased until essentially no further oxygen can be chemisorbed on the surface, at which the point the difference would become constant.

Activating radiation, i.e., bandgap energy light will create hole-electron pairs which, because of the potential difference between the bulk and the surface of the crystal, would become separated with the holes possibly combining with chemisorbed oxygen and thereby releasing the oxygen and leaving an electron trapped in the conduction band. if the light is turned off, the electron could either slowly recombine with atmospheric oxygen or, if placed in a reducible metal ion-containing solution, e.g. silver ion, could combine to form the elemental metal. There would be no electroneutrality imbalance since a negative ion of the solution, e.g. nitrate or hydroxyl ion, could be chemisorbed by the surface preserving electroneutrality of both solution and crystal surface.

The efficiency of the photographic properties of the photoconductor increases as the potential difference between the bulk and the surface increases until the nonstoichiometry becomes so great that substantially no further chemisorption is possible, and then electrons begin to pile up in the conduction band. These excess electrons decrease the efficiency due to the increased probability of combination of the holes with electrons before the holes could reach the chemisorbed oxygen for removal. As the excess electrons also reduce silver ion, the photographic response (speed) begins to decrease at about the point where fog would begin to develop.

Further, the return of oxygen to the surface after a light-effected change would be slower for materials with more oxygen already on the surface than for those with less of the surface covered, i.e. the image decay time increases with increasing bulk nonstoichiometry. Thus, the photographic response would be expected to increase, go through a maximum, and then decrease as the nonstoichiometry increases. In addition,

the image decay time increases as the nonstoichiometry increases.

The foregoing theoretical explanation of the phenomenon of this invention is merely offered to permit a better understanding of the invention and is not meant to be binding on the applicant.

In the practice of the invention, the photoconductor is heated in any suitable manner, e.g. as described in commonly assigned copending applications Ser. No. 463,037 filed June 10, 1965 now abandoned. in its broadest aspects, the practice of this invention involves heating the photoconductor at the selected elevated temperature and selected time period and rapidly quenching the heated photoconductor, i.e. cooling to a temperature below about 50 C. and preferably to or just below room temperature. For best results, the heated sample is quenched as rapidly as possible. Usually, a time period of up to about 2-3 minutes is practicable and gives acceptable results. After quench, the photoconductor is exposed to the ambient atmosphere to permit equilibration of the surface with oxygen which, as previously indicated, is quite rapid. The equilibration of the surface with atmospheric oxygen is preferred for obvious reasons, e.g. ease of handling, no requirement for special equipment, etc. Of course, the equilibration can be accomplished using any oxygen-containing atmosphere from which the surface of the photoconductor can absorb oxygen. Such an atmosphere can be introduced into the quench zone of the furnace apparatus described herein or, alternatively, the sample may be inserted into a suitable apparatus in which the desired oxygen atmosphere is maintained.

In the preferred fonn of the invention, the present process is carried out in a system which permits maintenance of the required atmosphere during the heating step. For example, a furnace system as shown in FIG. 1 may be used. The system is composed of a furnace zone (22) and a quench zone (23) with gas inlet (24) and outlet means (25). A receptacle (26) for the photoconductor sample, e.g., a platinum boat, is heated in the furnace zone (22) and, after heating, quenched in the quenching zone (23) by manipulation of means (27) for moving the receptacle (26). For equilibration of the surface of the heated photoconductor, the receptacle (26) is removed and exposed to atmospheric oxygen, or alternatively, the entire furnace is opened to the atmosphere and the sample thus exposed.

For the preferred photoconductor, titanium dioxide, the oxygen fugacity of the atmosphere in the sample chamber is controlled by using known mixtures of gases which will provide the desired partial pressure of oxygen. Especially suitable are mixtures of oxygen/ carbon dioxide and carbon monoxide/ carbon dioxide which provide fugacities over a wide range. The gas inlet and outlet means are both within the hot zone of the furnace which is preferably maintained at constant temperature (112 C.) over the entire sample to minimize variation. The constant temperature also allows to thermal separation of the gases which may occur were there a gradient between the inlet and outlet.

When carbon dioxide is used it is first purged of 0 before it is mixed with other gases, e.g. by passing it over copper tumings at elevated temperature. Pure carbon dioxide at 750 C. will yield an oxygen fugacity of l.8 l0"' atmospheres. When mixed with carbon monoxide, the oxygen fugacity can be lowered to l.7 l0 Mixing carbon dioxide with oxygen permits variation of the oxygen fugacity from 1X10 atmospheres.

The powdered photoconductor is placed in the receptacle and heated in the desired atmosphere until the desired extent of change in the stoichiometry occurs, after which the sample is quenched as described.

For best results, the samples are heated for a period of 24 hours although maximum stoichiometric change generally is achieved in shorter time. For example, heating titanium dioxide at 750 C. usually requires a time of about 16 hours, but heating for longer periods has no adverse effect on the samples. of course, heating in the reducing atmosphere has a beneficial effect on the photoconductor regardless of whether maximum change in stoichiometry is achieved, but, optimum results being most desirable, maximum alteration is almost invariably sought.

The heating of the photoconductor is conducted at a temperature of at least 500 C. with preferred temperatures in the range from about 700 C. to about 800 C. Temperatures higher than 800 C. are usually avoided since sintering of the photoconductor can occur at extreme temperatures and little advantage is gained.

After heating is completed, the receptacle containing the treated sample is drawn into a quench zone and brought to or near room temperature, and preferably slightly below room temperature. The quench zone may be, for example, watercooled and, desirably, the sample is cooled within a period of 2-3 minutes. The fast quenching of the sample minimizes or prevents the return of the mass to the original stoichiometry, e.g. reoxidation of the titanium dioxide bulk.

The starting material of the present invention is any form of the photoconductor which is suitable for use in image formation as described in the aforesaid copending applications. Usually, it is preferred to preheat the sample to remove volatile impurities. For example, samples of titanium dioxide were preheated to 650 C. for 3 hours to remove chloride and other volatile impurities.

After the sample of photoconductor is heated and quenched, the photoconductor is then used to make photosensitive media on which photographic evaluations are made, for example, handsheets of the sample materials by coating a film or paper with a dispersion of the photoconductor. Preferably, the dispersions are prepared using an ultrasonic probe for blending the dispersions. As dispersing agents, there may be employed sodium hexametaphosphate, potassium tripolyphosphate, and other dispersing agents known to the art. The preferred coating method employs a roll coater in which the coating is applied by pulling the paper rather than the rod.

The inert carrier on which the photoconductor is deposited comprises any suitable backing of sufficient strength and durability to satisfactorily serve the intended purpose. Reproduction carrier sheets can be in any form, such as, for example,

sheets, ribbons, rolls, etc. This sheet may be made of any suitable material such as wood, rag content paper, pulp paper, plastics such as, for example, polyethylene terephthalate and cellulose acetate, cloth, metallic sheets, such as an aluminum sheet, and glass. The preferred form of the carrier sheet is a thin sheet which is flexible and durable.

It is also useful to use a binder agent to bind the photoconductor to the carrier sheet. In general, these binders are translucent or transparent so as not to interfere with transmission of light therethrough. Preferred binder materials are organic materials such as resins. Examples of suitable resins are butadiene-styrene copolymers, poly(alkyl-acrylates) such as poly(methylmethacrylate), polyamides, poly-vinyl acetate, polyvinyl alcohol and polyvinylpyrrole.

The photoconductor may also be in the form of finely divided particles dispersed in a support, such as paper. The photoconductor may also be in the form of a continuous film, alone or in combination with a separate support.

Before photoexposure, the photoconductor is conditioned in the dark, generally for from 1 to 24 hours. After conditioning, the photoconductor is not exposed to light prior to its exposure to activating radiations for recording an image pattern.

The period of exposure will depend upon the intensity of the light source, the particular imaging material, the particular photoconductor, and like factors known to the art. in general, however, the exposure may vary from about seconds to several minutes.

Image-forming materials which are useful in this invention are those such as described in US. Pat. No. 3,152,903 and in copending application Ser. No. 199,21 1, now abandoned. These image-forming materials include preferably an oxidizing agent and a reducing agent. Such image-forming materials are often referred to in the art as physical developers. The oxidizing agent is generally the image-forming component of the image-forming material. However, this is not necessarily always the case. Either organic or inorganic oxidizing agents may be employed as the oxidizing component of the imageforming material. Preferred oxidizing agents comprise the reducible metal ions having at least the oxidizing power of cupric ion and include such metal ions as Ag Hg, Pb, Au, Pt, Ni, Sn, Pb, Cu, and Cu. Other suitable oxidizing agents useful in this invention as components of an image-forming material are permanganate (MnOf) ion, various leuco dye materials such as disclosed in copending application Ser. No. 623,534 filed in the name of L. Case, and the like. Organic oxidizing agents include tetrazolium salts, such as tetrazolium blue and red, and diphenyl carbazone, and genarcyl red 6B (methine dye).

The reducing agent components of the image-forming materials of this invention include organic compounds such as the oxalates, formates, substituted and unsubstituted hydroxylamine, and substituted and unsubstituted hydrazine, ascor' bic acid, aminophenols, and the dihydric phenols. Also, polyvinylpyrrolidone, alkali and alkaline earth metal oxalates and formates are useful as reducing agents. Suitable reducing compounds include hydroquinone or derivatives thereof, 0-

Additionally, the image-forming materials or physical developers may contain organic acids which can react with metal ions to form complex metal anions. Further, the developers may contain other complexing agents and the like to improve image formation and other properties found to be desirable in this art.

Additionally electrical toners may be used as image-forming materials in this invention.

Additional stabilizing and fixing steps such as known to the art may also be added to the processes of this invention in order to increase the life and permanence of the final print.

The following examples are illustrative of the present invention:

EXAMPLE 1 The determination of optimum conditions for obtaining optimum results is accomplished by routine experimentation utilizing a furnace unit as shown in FIG. 1. A sample of photoconductor is heated in the receptacle (26) at various fugacities of oxygen and then quenched to room temperature, and equilibrated with atmospheric oxygen, after which the photoresponse of the photoconductor is determined. A plot of the fugacity of oxygen versus the photographic speed will indicate the optimum fugacity for optimum photographic speed.

For example, samples of titanium dioxide were preheated at 650 C. for 3 hours and then were heated at 750 C. for 24 hours at different fugacities of oxygen, provided in the form of mixtures with gas(es). Samples heated beyond 16 hours show no change of photographic response. The inlet and outlet gases were both within the hot zone of the furnace which is maintained 12 C. over the entire sample chamber. After heating, the platinum receptacle boat was drawn into the quench zone (which was water cooled) and brought to room temperature, or slightly below, within 2-3 minutes, after which the samples were exposed to atmospheric oxygen for about 5-10 minutes.

Handsheets of the sample materials were then made and the photographic evaluations were made on a sensitometer using a 10" second exposure and a neutral density filter which is 1 .54 in the region of sensitivity of titanium dioxide. To ensure that the results were not being influenced by a spectral response shift, a Wratten 2A Cutoff filter is placed along one edge of the stepwedge used. (This filter cuts off at 405420 millimicrons). No response is detectable through this filter. To eliminate processing errors, samples were processed in freshly made solutions.

The results are tabulated in Table l and FIG. 2. The samples were processed in the same solution at the same time. Duplicately prepared samples treated in this way generally yielded reproducibility of 10.07 log units. From FIG. 2, as the theoretical explanation hereinbefore given would predict, the photographic response goes through a maximum and then falls off, and the decrease is about the point where the fog in the developed sample begins to increase.

Table 1 follows:

NOTE.Photographic speed=100/Eu-r; Reproducibility of duplicate run was 0.07 log units. The speed of the untreated titanium dioxide was 30-35.

and P-aminophenol, p-methylaminophenol sulfate, p-hydroxyphenyl glycine, o and p-phenylene diamine, and l-phenyl-3- pyrazolidone.

The decay time of the latent image formed on the samples subsequent to photoexposure was determined for samples l, 111 and Vll by processing at time intervals after exposure. The

results are tabulated in Table 2 and FIG. 3.

Table 2 follows:

TABLE 2 Time after exposure see. 1% hr. 22 hr. 27 hr. 51 hr. 125 hr. 312 hr.

Image density Similar results are observed with zinc oxide and indium oxidc.

EXAMPLE 2 Titanium dioxide different from that employed in example 1 was subjected to the same procedure as in example 1.

As the fugacity of oxygen was decreased from 1 to the optimum fugacity was determined at lXl0 The photographic response of the titanium dioxide increased as the fugacity decreased, with the maximum falling at 1Xl0*".

EXAMPLE 3 EXAMPLE 4 The procedure of example 3 is repeated using the titanium dioxide sample of example 2 equilibrated at a fugacity of oxygen of 1Xl0'". The resulting coated sheet is suitable for photographic use.

EXAMPLE 5 The titanium dioxide sample used in example 3 is dispersed in gelatin and the dispersion is coated on a cellulose triacetate sheet. The resulting sheet is suitable for photographic use.

Similarly, polyethylene terephthalate sheets are coated with the same gelatin dispersion to obtain useful photographic material.

The gelatin dispersion is prepared by forming a slurry of the photoconductor in water to which is added an aqueous gelatin solution. The plastic sheet is then coated with the gelatin dispersion by means of a wire wound rod.

It is known to improve the photographic properties of photoconductors by heating either in vacuo or in ambient atmosphere. Commonly assigned eopending application Ser. No. 463,037 filed June 10, 1965 in the name of Ronald Francis, now abandoned, describes the latter process. A comparison of the photographic speed of, for example, titanium dioxide activated in accordance with the disclosure of the saidcopending application by heating at 650 and 730 C. with sample [I of titanium dioxide described in example 1 of this application shows the latter to be markedly superior. The L.E.S. at both 0.1 and 0.2 is better by a factor of 2 over the sample heated at 730 C. and a factor of 3 over the sample heated at 650 C.

L.E.S. (abbreviation for light exposure speed) refers to a speed-rating system developed at the Wright Air Development Division of the Air Research and Development Command (U.S.A.F. and is defined as the reciprocal of the exposure in meter candle seconds which is required to produce a double diffuse reflection density of 0.2 density units above the sum of the base plus fog densitiesv As in the more conventional ASA system used to rate silver halide films. the higher the L.E.S. number the faster the photographic exposure speed of the film In the foregoing examples, the latent image is convened to a visible image by contact with a saturated solution of silver nitrate in methanol and then a solution of 5 g. of phenidone, 40 g. of citric acid monohydrate and l liter of methanol. The visible image-bearing print is fixed with methanolic potassium cyanate stop bath, then fixed in aqueous sodium thiosulfate solution and finally washed in water.

The spectral response of the products of the present invention may be altered bydoping with foreign metal ions such as chromium ions or by use of dyes, especially those which extend the spectral response into the visible region of the spectrum. Exemplary dyes are described in commonly assigned eopending application Ser. Nos. 633,689 filed Apr. 26, I967 and 623,534 filed March I9, 1967 What we claim is:

l. A process of improving the photographic properties of metal oxide photoconductors from the group consisting of titanium dioxide, zinc oxide and indium oxide having an average particle size of 250 millimicrons or less which comprises the steps of:

a. heating the photoconductor at a temperature above about 500 up to about 800 C. in a reducing atmosphere to produce an anion deficient or cation excess stoichiometry of the photoconductor,

b. rapidly quenching the heated photoconductor in the said atmosphere to a temperature below 50 C. to thereby retain the anion deficient or cation excess stoichiometry of the photoconductor, and,

c. equilibrating the quenched photoconductor with oxygen.

2. Process as in claim 1 wherein the atmosphere is a reducing atmosphere containing oxygen at a fugacity of less than one.

3. Process as in claim 1 wherein the atmosphere is a reducing atmosphere containing oxygen at a fugacity in the range of from about 1X10 to about 1Xl0 4. Process as in claim I wherein the temperature is at least 500 C.

5. Process as in claim 2 wherein the temperature is in the range of from about 700 to about 800 C.

6. Process as in claim 5 wherein the heating is conducted for a period of at least about 16 hours.

7. Process as in claim 1 wherein the photoconductor is titanium dioxide.

8. Process as in claim 1 wherein the photoconductor is zinc oxide.

9. Process of improving the photographic properties of titanium dioxide which comprises heating particulate titanium dioxide in a reducing atmosphere at a temperature of between about 500 and about 900 C. at an oxygen fugacity of less than one for a period of time sufficient to change the stoichiometry of the titanium dioxide in the bulk, rapidly quenching the heated titanium dioxide in said atmosphere to a temperature below about 50 C. and equilibrating the quenched titanium dioxide with oxygen.

10. Process as in claim 9 wherein the titanium dioxide is of an average particle size of about 250 millimicrons or less.

11. Process as in claim 9 wherein the temperature is in the range of from about 700 to about 750 C. and the fugacity of oxygen is in the range of from about 1X 1 0" to about 1X10 12. Process as in claim 11 wherein the heating is conducted for a period of at least about 16 hours and wherein the fugacity of oxygen is about IXIO'".

13. Process as in claim 11 wherein the atmosphere comprises carbon dioxide.

14. Process as in claim 11 wherein the atmosphere comprises a mixture of carbon monoxide and carbon dioxide.

[5. ln the process of improving the photographic properties of metal oxide photoconductors from the group consisting of titanium dioxide, zinc oxide and indium oxide having an average particle size of 250 millimicrons or less by heating at elevated temperatures, the improvement which comprises conducting the heating at a temperature of between about 500 up to about 800 C. in a reducing atmosphere to produce an anion deficient or cation excess stoichiometry of the photoconductor, rapidly quenching the heated photoconductor in the said atmosphere to a temperature below about 50 C. and equilibrating the quenched photoconductor in oxygen.

16. A metal oxide photoconductor, from the group consisting of titanium dioxide, zinc oxide and indium oxide the stoichiometry of the mass of which differs from the stoichiometry of the surface thereof to create a significant potential difference between said surface and said mass, and wherein said photoconductor has been treated according to the process of claim l5.

l7. Partially-reduced titanium dioxide having an oxygen deficient stoichiometry in the bulk and the surface of which is equilibrated with oxygen until substantially no further chemisorption of oxygen can occur, and wherein the titanium dioxide has been treated according to the process of claim l5.

18. An image reproduction system having improved photographic speed comprising a partially reduced metal oxide photoconductor from the group consisting of titanium dioxide, zinc oxide and indium oxide having an average particle size of 250 millimicrons or less and having an anion deficient or cation excessive stoichiometry in the bulk, the surface of which photoconductor is equilibrated with oxygen until substantially no further chemisorption can occur and wherein the anion deficiency or cation excess of the stoichiometry is not so great as to cause excessive background-fogging when the image reproduction system is exposed and contacted with a developer comprising silver nitrate, and wherein the photoconductor has been treated according to the process of claim 27.

19. An image reproduction system as in claim 18 wherein the photoconductor is titanium dioxide incorporated in a binder upon a suitable support.

20. An image reproduction system as in claim 18 wherein the photoconductor is in the form of a thin film.

21. An image reproduction system as in claim 18 wherein the photoconductor is in the form of finely divided particles dispersed in a support.

22. An image reproduction system as in claim 18 wherein the photoconductor is dye sensitized or doped with foreign lOllS.

23. An image reproduction system having improved photographic speed comprising titanium dioxide having improved photographic speed comprising titanium dioxide having an average particle size of about 250 millimicrons or less and having an oxygen deficient stoichiometry in the bulk, the surface of which titanium dioxide is equilibrated with oxygen until substantially no further chemisorption can occur, wherein the oxygen deficiency of the stoichiometry is not so great as to cause excessive background fogging when the image reproduction system is exposed and contacted with a developer comprising silver nitrate, and wherein the titanium dioxide has been treated according to the process of claim 17.

UNITED STATES PATENT OFFlCE CERTIFICATE OF COECIN Patent NO. 3,630 743 Dated December 28, 1971 Inventor(s) Martin L. Harvill It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 5 after "occur to a" insertsignificant- Column 8, V. line 26, before "50C" insert-about-; line 50, "900" should read800-. Column 10, line 2, "27" should read--l5-; and lines 15 and 16 the first mention of "having improved ghcitoggaphic speed comprising titanium dioxide" should be e ete Signed and sealed this 31st day of October 1972.

(SEAL) Attest:

EDWARD M.FLETGHER,JR ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents FORM PO-10 0 (10- USCOMM-DC 60376-P69 US. GOVERNMENT PRINTING OFFICE: 1969 0366-3!4 

2. Process as in claim 1 wherein the atmosphere is a reducing atmosphere containing oxygen at a fugacity of less than one.
 3. Process as in claim 1 wherein the atmosphere is a reducing atmosphere containing oxygen at a fugacity in the range of from about 1 X 10 1 to about 1 X 10
 20. 4. Process as in claim 1 wherein the temperature is at least 500* C.
 5. Process as in claim 2 wherein the temperature is in the range of from about 700* to about 800* C.
 6. Process as in claim 5 wherein the heating is conducted for a period of at least about 16 hours.
 7. Process as in claim 1 wherein the photoconductor is titanium dioxide.
 8. Process as in claim 1 wherein the photoconductor is zinc oxide.
 9. Process of improving the photographic properties of titanium dioxide which comprises heating particulate titanium dioxide in a reducing atmosphere at a temperature of between about 500* and about 800* C. at an oxygen fugacity of less than one for a period of time sufficient to change the stoichiometry of the titanium dioxide in the bulk, rapidly quenching the heated titanium dioxide in said atmosphere to a temperature below about 50* C. and equilibrating the quenched titanium dioxide with oxygen.
 10. Process as in claim 9 wherein the titanium dioxide is of an average particle size of about 250 millimicrons or less.
 11. Process as in claim 9 wherein the temperature is in the range of from about 700* to about 750* C. and the fugacity of oxygen is in the range of from about 1 X 10 1 to about 1 X 10
 20. 12. Process as in claim 11 wherein the heating is conducted for a period of at least about 16 hours and wherein the fugacity of oxygen is about 1 X 10
 17. 13. Process as in claim 11 wherein the atmosphere comprises carbon dioxide.
 14. Process as in claim 11 wherein the atmosphere comprises a mixture of carbon monoxide and carbon dioxide.
 15. In the process of improving the photographic properties of metal oxide photoconductors from the group consisting of titanium dioxide, zinc oxide and indium oxide having an average particle size of 250 millimicrons or less by heating at elevated temperatures, the improvement which comprises conducting the heating at a temperature of between about 500* up to about 800* C. in a reducing atmosphere to produce an anion deficient or cation excess stoichiometry of the photoconductor, rapidly quenching the heated photoconductor in the said atmosphere to a temperature below about 50* C. and equilibrating the quenched photoconductor in oxygen.
 16. A metal oxide photoconductor from the group consisting of titanium dioxide, zinc oxide and indium oxide, the stoichiometry of the mass of which differs from the stoichiometry of the surface thereof to create a significant potential difference between said surface and said mass, and wherein said photoconductor has been treated according to the process of claim
 15. 17. Partially-reduced titanium dioxide having an oxygen deficient stoichiometry in the bulk and the surface of which is equilibrated with oxygen until substantially no further chemisorption of oxygen can occur, and wherein the titanium dioxide has been treated according to the process of claim
 15. 18. An image-reproduction system having improved photographic speed comprising a partially reduced metal oxide photoconductor from the group consisting of titanium dioxide, zinc oxide and indium oxide having an average particle size of 250 millimicrons or less and having an anion deficient or cation excessive stoichiometry in the bulk, the surface of which photoconductor is equilibrated with oxygen until substantially no further chemisorption can occur and wherein the anion deficiency or cation excess of the stoichiometry is not so great as to cause excessive background-fogging when the image reproduction system is exposed and contacted with a developer comprising silver nitrate, and wherein the photoconductor has been treated according to the process of claim
 27. 19. An image reproduction system as in claim 18 wherein the photoconductor is titanium dioxide incorporated in a binder upon a suitable support.
 20. An image reproduction system as in claim 18 wherein the photoconductor is in the form of a thin film.
 21. An image reproduction system as in claim 18 wherein the photoconductor is in the form of finely divided particles dispersed in a support.
 22. An image reproduction system as in claim 18 wherein the photoconductor is dye sensitized or doped with foreign ions.
 23. An image reproduction system having improved photographic speed comprising titanium dioxide having an average particle size of about 250 millimicrons or less and having an oxygen deficient stoichiometry in the bulk, the surface of which titanium dioxide is equilibrated with oxygen until substantially no further chemisorption can occur, wherein the oxygen deficiency of the stoichiometry is not so great as to cause excessive background fogging when the image reproduction system is exposed and contacted with a developer comprising silver nitrate, and wherein the titanium dioxide has been treated according to the process of claim
 15. 